A number of cosmetic formulations consist mostly of water and a non-aqueous phase, in particular with a base of oils, waxes, surfactants, solvents or other excipients. These include creams, foaming agents, makeup products, etc. that may be designated by the expression cosmetic formulations for skin application, in the sense that they are intended to be applied to the skin. Quite naturally, an attempt is made to thicken such formulations in the pH range corresponding to that of the skin, i.e. values between 5 and 7, and preferentially between 5 and 6.5, very preferentially between 5.5 and 6.
There are a certain number of technical solutions to this problem which can be classified into 4 categories: the implementation of polymers of high molecular weight and in the powder form, the so-called “back acid” technique based on acrylic polymers in direct emulsion of polymer particles in water, the implementation of other polymers always in the form of direct emulsions, and finally the use of water-in-oil emulsions.
In the first category, reference can be made to document EP 1 138 703 A1 which describes a topical cosmetic composition including a high molecular weight polymer based on at least one monomer possessing a strong free acid function copolymerized with at least one esterified monomer and terminated by a hydrophobic group with 8 to 30 carbon atoms. The above-noted polymer is an emulsifying polymer in the solid form; it can be dispersed in water, and it enables the thickening of the composition that contains it, notably for pH values in the neighbourhood of 5.
However, these polymers have the disadvantages of the use of a powder: difficulties with transportation and cleaning, dangerousness of the product in relation to its powdery, irritating and particulate character. In addition, these polymers must be made soluble in the medium to be thickened by the introduction of surfactants. These surfactants are additional formulation additives that make the formulation more complex and can interact with surfactants already contained in the said formulation, sometimes creating adverse effects (including phase separation, formation of residual insolubles).
Also known is the so-called “back-acid” technique, as described in document WO 01/76 552. This is a process consisting of introducing a surfactant and an inflatable acrylic alkali copolymer into an aqueous medium. The latter leads to a thickening effect when its carboxylic acid groups are neutralized: There is then the creation of a three-dimensional network which leads to an increase in the viscosity of the aqueous phase. Such an effect can be triggered in a pH range in the neighbourhood of 6, the role of the surfactant being to maintain the thickening effect even when the pH is decreased.
To the above-noted mechanism can be added an associative mechanism based on the presence of a hydrophobic monomer: This is what is described by document WO 03/62 288 which also aims at thickening acidic pH formulations. The same is true for document U.S. Pat. No. 4,529,773 A1. As with the back-acid method, the presence of a surfactant in the form of an additional product is therefore necessary, resulting in the disadvantages already mentioned.
Also known are a certain number of documents that describe the implementation of other polymers in emulsion. As such, document EP 0 824 914 B1 describes a polymer containing a cationic amino monomer. The desired thickening effect will be obtained at an acidic pH via the ionization of the cationic amino monomer. In document WO 2004/024 779, the cationicity of the proposed polymer is provided by an amino-substituted vinyl monomer. Thickening an acidic pH aqueous medium is also achieved here. However, the toxicity of cationic polymers to aquatic wildlife is well known: now they are unfortunately found at the end of the life cycle in our rivers and tributaries into which they are discharged via the domestic wastewater system.
Finally, water-in-oil emulsions and their applications as thickening agents are known in the field of cosmetics, as disclosed in documents WO 2004 063228 A1 and GB 2 422 605 A1. Nevertheless, these structures require the presence of surfactants and solvents to ensure their stability, and the disadvantages mentioned above are then encountered.